Low Forward Voltage Rectifier

ABSTRACT

A Low Forward Voltage Rectifier (LFVR) includes a bipolar transistor, a parallel diode, and a base current injection circuit disposed in an easy-to-employ two-terminal package. In one example, the transistor is a Reverse Bipolar Junction Transistor (RBJT), the diode is a distributed diode, and the base current injection circuit is a current transformer. Under forward bias conditions (when the voltage from the first package terminal to the second package terminal is positive), the LFVR conducts current at a rated current level with a low forward voltage drop (for example, approximately 0.1 volts). In reverse bias conditions, the LFVR blocks current flow. Using the LFVR in place of a conventional silicon diode rectifier in the secondary of a flyback converter reduces average power dissipation and increases power supply efficiency.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of, and claims priority under 35U.S.C. §120 from, nonprovisional U.S. patent application Ser. No.14/149,746 entitled “Low Forward Voltage Rectifier,” filed on Jan. 7,2014, the subject matter of which is incorporated herein by reference.Application Ser. No. 14/149,746, in turn, is a continuation of, andclaims priority under 35 U.S.C. §120 from, nonprovisional U.S. patentapplication Ser. No. 13/317,800 entitled “Low Forward VoltageRectifier,” filed on Oct. 29, 2011, now U.S. Pat. No. 8,649,199, thesubject matter of which is incorporated herein by reference.

TECHNICAL FIELD

The described embodiments relate to rectifiers, and more particularly torectifiers for flyback power supplies.

BACKGROUND INFORMATION

FIG. 1 (Prior Art) is a simplified circuit diagram of a flybackconverter power supply 1. Flyback converter 1 generates a 5.0 volt DCvoltage from a 110 volt AC source. 110 volts AC supplied from source 2is present between connectors 3 and 4. The 110 volt AC voltage isrectified by a full wave bridge rectifier comprising diodes 5-8.Capacitor 9 is a smoothing capacitor. A rough DC voltage V_(IN) ofapproximately the 150 volt peak voltage of the 110 volt AC RMS inputsignal is present on conductor and node 10. A switch 11 is rapidlyswitched on and off to pull pulses of current through the primarywinding 12 of a transformer 13 from this V_(IN) conductor. When a pulseof current is pulled through the primary winding 12, an amount of energyis stored in the transformer 13. When the switch 11 is then opened, apulse of current is made to flow from the secondary winding 14 so thatenergy stored in the transformer 13 is transferred to the load 15. Thecurrent from the secondary winding 14 flows through a rectifier diode16. Such pulses of current keep charge on a storage capacitor 17 so thatthe desired 5.0 volts DC is maintained across load 15 between V_(OUT)conductor 18 and ground conductor 19. Standard sensing and controlcircuitry that controls the switching of switch 11 is not illustrated inorder to simplify the diagram. The flyback topology of FIG. 1, includingits sensing and control circuitry, is well known in the art.

FIG. 2 (Prior Art) is a set of simplified waveform diagrams. Thesediagrams set forth waveforms of voltages and currents present in thecircuit of FIG. 1. The upper waveform labeled V_(S) shows the voltagepresent across switch 11. From time t₀ to time t₁, switch 11 is closed.Current is flowing from node 10, through the primary winding 12, andthrough switch 11, and to ground node and conductor 20. From time t₀ totime t₁ this current increases as illustrated in the waveform labeledI_(S). From time t₀ to time t₁, energy is being stored in thetransformer. Switch 11 is closed. Accordingly, the voltage across switch11 is zero. Magnetic flux is building in the transformer as indicated bythe waveform labeled “MAGNETIC FLUX”. Then at time t₁, switch 11 isopened. The opening of switch 11 causes a current to stop flowing in theprimary winding 12, and to start flowing in the secondary winding 14. Asillustrated in the fourth waveform labeled I_(D), this current flowingin the secondary winding 14 decreases over time. The magnetic flux inthe transformer decreases as well. At time t₂, there is no more energystored in the transformer and the secondary current stops flowing. Fromtime t₂ to time t₃, there is little or no current flow in either theprimary winding 12 or the secondary winding 14 of the transformer asindicated by the I_(S) and I_(D) waveforms. The switching cycle repeatsat time t₀ when switch 11 is closed again to start the next cycle. Theswitching period from time t₀ of one period to time t₀ of the nextperiod may, for example, be ten microseconds.

FIG. 3 (Prior Art) illustrates current flow from time t₂ to time t₀.Reference numeral 21 identifies the split core of transformer 13. FIG. 4(Prior Art) illustrates current flow from time t₀ to time t₁. FIG. 5(Prior Art) illustrates current flow from time t₁ to time t₂.

When current is flowing from the secondary winding 14 of the transformer13 and to capacitor 17 and load 15, the current is flowing throughrectifier diode 16. The rectifier diode 16 being in the current pathresults in unwanted power dissipation. At a given time, theinstantaneous power dissipated in rectifier diode 16 is the product ofthe instantaneous current flow through the diode and the instantaneousvoltage being dropped across the diode. Average power dissipation inrectifier diode 16 is the average of such instantaneous powerdissipation taken over the entire switching cycle of the flybackconverter. In a common conventional flyback converter that outputs 20amperes at 5.0 volts DC such as the flyback converter illustrated inFIG. 1, the forward voltage drop V_(F) of the rectifying diode at itsrated current flow is approximately 1.0 volts. Average power dissipationin the rectifying diode may be approximately 15 Watts.

SUMMARY

A Low Forward Voltage Rectifier (LFVR) in an easy-to-employ two-terminalTO-247 package comprises a first package terminal, a second packageterminal, a bipolar transistor, a parallel diode, and a base currentinjection circuit. The collector of the bipolar transistor is coupled tothe first package terminal. The emitter of the bipolar transistor iscoupled to the second package terminal. The parallel diode is coupledbetween the collector and the emitter of the bipolar transistor so thatthe anode of the diode is coupled to the collector and the cathode ofthe diode is coupled to the emitter. The base current injection circuitinjects a base current into the bipolar transistor in forward biasconditions (conditions in which the voltage on the first packageterminal is greater than the voltage on the second package terminal)such that the voltage drop from the first package terminal to the secondpackage terminal is substantially less than 0.7 volts when a currentgreater than a predetermined current is flowing from the first packageterminal to the second package terminal. The forward voltage drop fromthe first to second package terminals may be approximately 0.1 volts.

In one example, if current flow from the first package terminal to thesecond package terminal is less than the predetermined current underforward bias conditions, then the voltage drop from the first packageterminal to the second package terminal is limited by the diode to beapproximately 0.8 volts. In reverse bias conditions (conditions in whichthe voltage between the first package terminal and the second packageterminal is negative), the LFVR blocks current flow.

In one example, the base current injection circuit involves a currenttransformer. The current transformer has a first winding and a secondwinding wrapped around a ring-shaped ferrite core. The currenttransformer, the bipolar transistor, and the parallel diode areinterconnected such that the second winding is in the current path ofthe collector current of the bipolar transistor. The first winding iscoupled to supply a base current to the base of the bipolar transistor.In one specific case, the base current I_(B) supplied by the firstwinding of the current transformer to the base is approximately onethird of the collector current I_(C) flowing through the second windingof the transformer. For collector currents greater than thepredetermined critical collector current I_(C-CRIT), the base currentsupplied to the bipolar transistor is adequate to keep the transistor insaturation such that V_(CE) is substantially less than 0.7 volts(approximately 0.1 volts).

In one example, the bipolar transistor is a Reverse Bipolar JunctionTransistor (RBJT) and the parallel diode is a distributed diode. Boththe RBJT and the distributed diode are parts of a novel integratedcircuit. The reverse breakdown voltage from the emitter of the RBJT tothe base of the RBJT is greater than 20 volts. The reverse breakdownvoltage from the emitter of the RBJT to the collector of the RBJT isgreater than 20 volts. Using the Low Forward Voltage Rectifier for therectifying component in a flyback converter power supply reduces averagepower dissipation as compared to using a conventional silicon diode forthe rectifying component. Reducing average power dissipation increasespower supply efficiency. The easy-to-employ two-terminal TO-247 packageof the LFVR allows a conventional diode in the secondary of a flybackconverter to be removed and replaced with the LFVR with a minimal amountof PCB layout and power supply design changes.

Further details and embodiments and techniques are described in thedetailed description below. This summary does not purport to define theinvention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, where like numerals indicate like components,illustrate embodiments of the invention.

FIG. 1 (Prior Art) is a diagram of a conventional flyback converterpower supply.

FIG. 2 (Prior Art) is a waveform diagram showing voltages and currentsin the circuit of FIG. 1.

FIG. 3 (Prior Art) illustrates current flow in the conventional flybackconverter of FIG. 1 from time t₂ to time t₀.

FIG. 4 (Prior Art) illustrates current flow in the conventional flybackconverter of FIG. 1 from time t₀ to time t₁.

FIG. 5 (Prior Art) illustrates current flow in the conventional flybackconverter of FIG. 1 from time t₁ to time t₂.

FIG. 6 is a flyback converter that includes a Low Forward VoltageRectifier (LFVR) in accordance with one novel aspect.

FIG. 7 is a diagram of the IV characteristic of a silicon diode.

FIG. 8 is a diagram of the IV characteristic of a Schottky diode.

FIG. 9 is a diagram of the V_(CE) to I_(C) characteristic of aconventional BJT, provided that an adequately large base current issupplied to the BJT.

FIG. 10 is a diagram of the V_(CE) to I_(C) characteristic of a ReverseBipolar Junction Transistor (RBJT), provided that an adequately largebase current is supplied to the RBJT.

FIG. 11 is a table that sets forth V_(F) and V_(T) for various differenttypes of rectifying components.

FIG. 12 is a top-down schematic diagram of a square portion of anintegrated circuit in accordance with one novel aspect. The integratedcircuit includes both the RBJT and the distributed parallel diode inintegrated form.

FIG. 13 is a cross-sectional side view taken along sectional line A-A ofthe square portion of FIG. 12.

FIG. 14 is a diagram of the fly-back converter of FIG. 6, but with aspecific implementation of the LFVR.

FIG. 15 is a set of waveform diagrams that shows voltages and currentsin the fly-back converter of FIG. 14.

FIG. 16 illustrates current flow in the circuit of FIG. 14 from time t₂to time t₀.

FIG. 17 illustrates current flow in the circuit of FIG. 14 from time t₀to time t₁.

FIG. 18 illustrates current flow in the circuit of FIG. 14 from time t₁to time t₂.

FIG. 19 is a simplified cross-sectional diagram of a conventionalbipolar junction transistor (BJT) structure.

FIG. 20 is a simplified cross-sectional diagram of the RBJT of FIG. 14.

FIG. 21 is a waveform diagram showing the voltage drop across andcurrent flow through a conventional diode if the conventional diode isused for the rectifying component in a flyback converter.

FIG. 22 is a waveform diagram showing the voltage drop across andcurrent flow through the LFVR of FIG. 14 between time t₁ and time t₂.

FIG. 23 is a table that compares the average power dissipation of theconventional flyback converter circuit of FIG. 1 involving aconventional silicon diode as the rectifying component to the averagepower dissipation of the novel flyback converter circuit of FIG. 6involving an LFVR as the rectifying component.

FIG. 24 is a detailed diagram of a particular implementation of the LFVRof FIG. 14.

FIG. 25 is a simplified perspective conceptual diagram of the currenttransformer of the LFVR of FIG. 14.

FIG. 26 is a more detailed perspective diagram of the currenttransformer of the LFVR of FIG. 14.

FIG. 27 is a simplified perspective view of the packaged LFVR of FIG. 14before encapsulation.

FIG. 28 is a simplified perspective view of the packaged LFVR of FIG. 14after encapsulation.

FIG. 29 is a simplified flowchart of a method 300 in accordance with onenovel aspect.

DETAILED DESCRIPTION

Reference will now be made in detail to background examples and someembodiments of the invention, examples of which are illustrated in theaccompanying drawings.

FIG. 6 is a simplified circuit diagram of a flyback converter 50 inaccordance with one novel aspect. Flyback converter 50 generates a 5.0volt DC (direct current) voltage from a 110 volt AC (alternativecurrent) source (110 VAC RMS). 110 volts AC RMS supplied from source 52is present between connectors 53 and 54. The 110 volt AC voltage isrectified by a full wave bridge rectifier comprising diodes 55-58.Capacitor 59 is a smoothing capacitor. A somewhat rough DC voltageV_(IN) is present on conductor and node 60. The magnitude of V_(IN) isapproximately the peak voltage of the 110 VAC RMS signal, which is 150volts. A switch 61 is rapidly switched on and off to pull pulses ofcurrent through the primary winding 62 of a transformer 63. When onesuch pulse of current is pulled from node 60, through the primarywinding 62, through switch 61, and to ground node and conductor 51, anamount of energy is stored in the transformer. When switch 61 is thenopened current stops flowing in the primary winding 62, but a pulse ofcurrent is then made to flow from the secondary winding 64 so thatenergy previously stored in the transformer is then transferred to theload 65. The current from the secondary winding 64 flows through a novelLow Forward Voltage Rectifier 66 (LFVR). Such pulses of secondarycurrent keep a storage capacitor 67 charged so that the desired 5.0volts DC is maintained across load 65 between V_(OUT) conductor 68 andground conductor 69. Sensing and control circuitry that controls theswitching of switch 61 is not illustrated in order to simply thediagram. Many suitable sensing and control circuits for controllingswitch 61 are known in the art.

In the illustrated example, LFVR 66 comprises a first package terminal70, a second package terminal 71, a base current injection circuit 72, aReverse Bipolar Junction Transistor (RBJT) 73, and a parallel diode 74,interconnected as illustrated in FIG. 6. The anode 75 of the diode 74 iscoupled to the collector of RBJT 73. The cathode 76 of the diode 74 iscoupled to the emitter of RBJT 73. The voltage from the first packageterminal 70 to the second package terminal 71 is denoted V_(LFVR). Thecurrent flow from the first package terminal 70 to the second packageterminal 71 is denoted I_(LFVR). The base current injection circuit 72functions to inject an adequate base current I_(B) into RBJT 73 so thatwhen current I_(LFVR) is flowing the voltage drop (from terminal 70 toterminal 71) is substantially lower than 0.7 volts (for example, about0.1 volts) throughout as much of the time from time t₁ to time t₂ aspossible.

FIG. 7 is a diagram showing the IV curve for an ordinary silicon diodewhere the junction is a semiconductor-semiconductor junction. The diodestarts to conduct current when a positive voltage of approximately 0.7volts (denoted V_(T)) is present from its anode to its cathode. When apositive voltage is present across the diode from its anode to itscathode, and the diode is conducting at its rated current, then theforward voltage drop across the diode is about 1.0 volts. This voltagedrop is denoted V_(F). When the diode is reverse biased (a negativevoltage is present from its anode to its cathode), then the diodeeffectively blocks current flow for negative voltages that are not toohigh. This is the type of diode commonly used for the rectifier diode ina conventional flyback converter.

FIG. 8 is a diagram showing the IV curve for another type of diodereferred to as a Schottky diode where the junction is ametal-semiconductor junction. As shown, the Schottky diode begins toconduct current at a lower positive voltage V_(T) of 0.4 volts betweenits anode and its cathode. The Schottky diode has a lower forwardvoltage V_(F) of 0.7 volts at the rated current of the diode.

FIG. 9 is a diagram showing the collector-to-emitter voltage V_(CE) dropacross an ordinary bipolar transistor as a function of collector currentI_(C). The curve of FIG. 9 assumes that the base current I_(B) isadequately large to keep the transistor in saturation. In the example,the base current I_(B) is one tenth of the rated collector current. Notethat the voltage drop V_(CE) is about 1.0 volts at the rated collectorcurrent.

If the silicon diode of FIG. 7 were used as the rectifier component inthe secondary of a flyback converter, then there would be a 1.0 voltagedrop across the diode when a pulse of secondary current is flowingthrough the diode at the diode rated current. This voltage drop wouldcorrespond to a high power dissipation. Similarly, if the Schottky diodeof FIG. 8 were used as the rectifier component in the secondary of aflyback converter, then there would be a 0.7 voltage drop across thediode when a pulse of secondary current is flowing through the diode atthe diode rated current. This voltage drop would correspond to anundesirably high power dissipation as well. Similarly, if the ordinarybipolar transistor of FIG. 9 were used as the rectifier component in thesecondary of a flyback converter, then there would be a 1.0 V_(CE)voltage drop across the transistor when the pulse of secondary currentis flowing through the transistor at the rated current. This 1.0 voltV_(CE) voltage drop would correspond to a high power dissipation.

FIG. 10 is a diagram showing the collector-to-emitter voltage V_(CE)drop across RBJT 73 of the Low Forward Voltage Rectifier (LFVR) 66 ofFIG. 6, where the base current I_(B) is maintained at one third of thecollector current I_(C). RBJT 73 starts conducting collector currentI_(C) at a collector-to-emitter voltage V_(CE) of about 0.7 volts. Asthe collector current increases, the V_(CE) across RBJT 73 increases upto about 0.9 volts for very low collector currents. As the collectorcurrent increases further, however, the V_(CE) across the transistordecreases rapidly. For a collector current equal to one third of therated collector current I_(RATED), the V_(CE) is less than 0.1 volts.Further increases in collector current I_(C) up to the RBJT ratedcollector current I_(RATED) only slightly increases the V_(CE) acrossthe RBJT. For a collector current equal to the rated collector currentI_(RATED), the V_(CE) is approximately 0.1 volts (denotes forwardvoltage V_(F)) as illustrated. In the illustrated example, I_(RATED) is30 amperes.

FIG. 11 is a table that sets forth the forward bias voltages V_(T) wherethe various rectifiers begin to conduct forward current. FIG. 11 alsoshows the voltage drops V_(F) across the various rectifiers when therectifiers are conducting at their rated currents.

FIG. 12 is a simplified top down diagram of a square portion 77 of RBJT73 and parallel diode 74. RBJT 73 and parallel diode 74 are integratedto be parts of the same integrated circuit 78. The square portion 77illustrated in FIG. 12 is replicated many times in rows of adjacentsquares and columns of adjacent squares across the integrated circuit78. From a top-down perspective, the base contact 79 has atwo-dimensional grid pattern of horizontally extending strips of thebase contact and vertically extending strips of the base contact. Withineach of the squares formed by this two-dimensional grid is a squareN-type collector region. Integrated circuit 78 involves approximatelyone hundred copies of the square illustrated in FIG. 12.

FIG. 13 is a cross-sectional side view taken along sectional line A-A inFIG. 12. A base metal electrode 80 makes electrical contact with P-typelayer 81 at base contact 79. A part 82 of the P-type layer 81 serves asthe P-type base region of RBJT 73, and another part 83 of the P-typelayer serves as the P-type anode of a PN junction 84. In a case wherelightly doped region 89 is P-type the actual PN junction of the diodewill be the interface between regions 89 and 90, whereas in a case wherelightly doped region 89 is N-type the actual PN junction of the diodewill be the interface between regions 81 and 89. N-type collectorregions 85 and 86 extend down into the P-type layer 81 from the uppersurface of the semiconductor material. A collector metal electrode 87makes contact with these N-type collector regions at a collector contact88. The collector metal electrode 87 also serves as an anode metalelectrode 95 and makes contact with the P-type anode 83 of PN junction84 at a diode contact 96. The collector metal electrode 87 bridges overthe base metal electrode 80 as illustrated. An amount of insulativematerial 120 prevents the collector metal electrode 87 from makingelectrical contact with the underlying base metal electrode 80. Thisdouble metal electrode structure involving a collector metal electrodethat bridges over a base metal electrode allows the RBJT to have a lowerforward voltage V_(F) as compared to a single metal layer structureinvolving interdigitated base and collector electrodes.

A lightly doped layer 89 is disposed under the P-type layer 81. AnN-type layer 90 is disposed under the lightly doped layer 89. A part 91of the N-type layer 90 serves as the emitter region of RBJT 73, whereasanother part 92 of the N-type layer 90 serves as the N-type cathode 92of PN junction 84. The entire bottom surface of the semiconductormaterial is covered with a layer of metal that serves as an emittermetal electrode 93 and as a cathode metal electrode 94. The simplifiedillustration of RBJT 73 involves N-type collector region 121, P-typebase region 82, an amount of the lightly doped region, and N-typeemitter region 91.

Note that each diode contact (a contact from metal electrode 87 and 95down to a PN junction below) appears as a circle in the top-downperspective of FIG. 12. The parallel diode 74 illustrated as a symbol inFIG. 6 actually comprises many PN junctions, each having a separatecircle-shaped diode contact. PN junction 84 of FIG. 13 is one of thesemany PN junctions. These many PN junctions are distributed across theintegrated circuit. This structure involving many PN junctionsdistributed across the integrated circuit is referred to as adistributed diode structure. The distributed diode structure providesbetter heat balancing as compared to a structure where the paralleldiode is realized as a single non-distributed junction that is locatedin only one localized part of the integrated circuit.

FIG. 14 is a diagram of the fly-back converter 50 of FIG. 6 but with aspecific implementation of LFVR 66 shown. The implementation of LFVR 66of FIG. 14 involves a current transformer 97 and the integrated circuit78. Current transformer 97 includes a first winding 98, a second winding99, and a ferrite core 100. The number of turns of the first winding 98is at least twice as large (for example, three times as large) as thenumber of turns of the second winding 99. As indicated by the dots onthe ends of the winding symbols in FIG. 14, the first and secondwindings 98 and 99 are wound with respect to one another such that anincrease in current in the second winding results in an increase incurrent in the first winding.

In the example of FIG. 14, the only electrical circuit component in thepath of the collector current is an inductive element (the secondwinding of current transformer 97). There is no resistive or diode orvoltage drop current sense element disposed in the collector currentpath between the first package terminal 70 and the collector of RBJT 73.There is no semiconductor material disposed in the collector currentpath. Similarly, there is no resistive or diode or voltage drop currentsense element disposed in the current path between the emitter of RBJT73 and the second package terminal 71. There is no semiconductormaterial disposed in the emitter current path.

FIG. 15 is a set of waveform diagrams that shows voltages and currentsin the fly-back converter 50 of FIG. 14. Switch 61 is closed from timet₀ to time t₁, and is open from time t₂ to time t₂ and then to time t₀of the next switching cycle. When the switch 61 is closed energy isbeing stored in the transformer 63, and when the switch 61 is open theenergy is transferred from the transformer 63 to capacitor 67 and load65. When the switch 61 is open and current is flowing through thesecondary winding 64, the forward voltage across LFVR 66 is about 0.1volts for most of the time from time t₂ to time t₂.

FIG. 16 illustrates current flow from time t₂ to time t₀. Referencenumeral 101 identifies the split core of transformer 63. FIG. 17illustrates current flow from time t₀ to time t₁. FIG. 18 illustratescurrent flow from time t₂ to time t₂.

FIG. 19 is a simplified cross-sectional diagram of a conventionalbipolar junction transistor (BJT) structure. The low doped region 200 isthe biggest source of conduction loss. The major charge carriers in thelow doped region 200 are electrons from the emitter 201. Holes from thebase 202 can enter the low doped region 200, but because the emittervoltage is lower than the voltage of the collector 203, the holes movetoward the emitter 201. Because the supply of holes in the low dopedregion is weak and because charge neutrality must be maintained, it isdifficult for the density of charge carriers in the low doped region 200to be much higher than the doping concentration of the low doped region.Accordingly, the low doped region has a relatively high resistance. Therelatively high resistance increases energy loss when the conventionalBJT is conducting current at its rated current.

FIG. 20 is a simplified cross-sectional diagram of RBJT 73. The lowdoped region 89 has more charge carriers than the low doped region 200in the conventional BJT structure of FIG. 19. There are both holes andelectrons in low doped region 89. Holes enter the low doped region fromthe base 82, and electrons enter the low doped region from the emitter91. Because the charges of these charge carriers are opposite oneanother, the charges cancel each other and net charge neutrality in lowdoped region 89 is maintained. Charge carrier density in the low dopedregion 89 is substantially higher than the doping concentration of thelow doped region. Accordingly, the low doped region has a relatively lowresistance and this relatively low resistance helps keep energy loss lowwhen the RBJT is conducting current at its rated current.

FIG. 21 is a waveform diagram for a conventional diode operating as therectifier component in a flyback converter from time t₁ to time t₂. Thetime from time t₁ to time t₂ is the time when current flows through thesecondary winding of the transformer and through the rectifyingcomponent. After an initial transient period, the voltage drop acrossthe forward biased diode decreases from about 1.0 volts to about 0.7volts at the end of the time period.

FIG. 22 is a waveform diagram for RBJT 73 of LFVR 66 of FIG. 14. Afteran initial transient period from t₁ to time t_(1A), the V_(CE) voltagedrop across RBJT 73 decreases from about 0.1 volts to about 0.05 volts.During this time from time t_(1A) to time t_(1B), the collector currentI_(C) and the base current I_(B) decrease as illustrated in the upperwaveform. At some point, the collector current reaches a criticalcurrent I_(C-CRIT) at which the base current I_(B) is not adequate tokeep the RBJT in saturation. This is indicated to occur at time t_(1B)in FIG. 22. At this time, V_(CE) begins increasing. At time t_(1C) theforward bias across diode 74 is large enough that diode 74 beginsconducting appreciable current. Diode 74 limits the voltage V_(LFVR) toabout 0.8 volts. At time t₂ current flow through the secondary winding64 stops, so current flow through LFVR 66 stops as well.

FIG. 23 is a table that compares the average power dissipation of theconventional flyback converter circuit of FIG. 1 (20 amperes out at 5.0volts DC) involving a conventional silicon diode as the rectifyingcomponent to the average power dissipation of the novel flybackconverter circuit of FIG. 6 (20 amperes out at 5.0 volts DC) involvingLFVR 66 as the rectifying component. Other than the type of rectifyingcomponent, the circuit topologies of the circuits of FIG. 1 and FIG. 6are identical.

FIG. 24 is a more detailed diagram of a particular implementation of theLFVR 66 of FIG. 14. A first end 102 of first winding 98 and a first end103 of second winding 99 are coupled together and to a transformerterminal 104. A second end 105 of first winding 98 is coupled to atransformer terminal 106. A second end 107 of second winding 99 iscoupled to a transformer terminal 108. The first winding 98 has threeturns. The second winding 99 has one turn. The terminal 104 of thetransformer is coupled via bond wire 109 to the first package terminal70 of LFVR 66. The terminal 106 of the transformer is coupled via bondwire 110 to a base terminal 111 of integrated circuit 78. The terminal108 of the transformer is coupled via bond wire 112 to a collectorterminal 113 of integrated circuit 78. An emitter terminal 114 (emittermetal electrode 93) of integrated circuit 78 is coupled via conductiveheat sink 115 to second package terminal 71. The conductive heat sink115 is a portion of the metal leadframe of the package. The secondpackage terminal 71 is a pin-shaped extension of the heat sink. Both thepin-shaped second package terminal 71 and the conductive heat sink 115are stamped from the same piece of leadframe metal. In the diagram, Aidentifies first package terminal 70: B identifies second packageterminal 71; C, D and E identify the three terminals of currenttransformer 97; F, G and H identify the three terminals of integratedcircuit 78. In the illustrated example, LFVR 66 has no package terminalother than the two package terminals 70 and 71.

FIG. 25 is a simplified perspective conceptual diagram of currenttransformer 97.

FIG. 26 is a more detailed perspective diagram of current transformer100. An insulative spacer 116 attaches to the bottom of the ring-shapedcore 100 as illustrated. The ring-shaped core 100 has bridging portions100A and 100B that allow turns of wire to loop under and around the corebetween the bottom of the bridge portions and the top of the spacer 116.In one example, insulative spacer 116 is a preformed injection moldedplastic part. In another example, insulative spacer 116 is heatconductive ceramic. In another example, insulative spacer 116 is a pieceof two-sided insulative tape.

FIG. 27 is a simplified perspective view of the packaged Low ForwardVoltage Rectifier (LFVR) 66 before encapsulation. The extension 117 ofthe stamped and formed copper leadframe is cut off. A piece ofinsulative tape 118 is disposed underneath the transformer 97 betweenthe transformer 97 and conductive heat sink 115. In this case theinsulative tape 118 is optional due to insulative spacer 116 beingpresent.

FIG. 28 is a perspective view of the packaged LFVR 66 after the assemblyhas been overmolded with an injection molded plastic encapsulant 119.The packaged LFVR 66 conforms to the form factor of a standard largeform factor through hole TO-247 package except that the middle terminalof the standard TO-247 package is not present. The package includes thetwo package terminals 70 and 71, the conductive heat sink 115, and theencapsulant 119.

FIG. 29 is a flowchart of a method 300 of manufacturing a packagedelectronic device. A LFVR (Low Forward Voltage Rectifier) is fabricated(step 301) by assembling a current transformer, a bipolar transistor,and a parallel diode such that the current transformer supplies anadequate base current to the transistor (for example, I_(B)=I_(C)/3)when current flows (under forward bias conditions) from the firstpackage terminal of the LFVR to the second package terminal of the LFVR,thereby causing the transistor to have a collector-to-emitter voltagesubstantially less than 0.7 volts. In one example, the assembled LFVR isthe LFVR 66 illustrated in FIGS. 24, 27 and 28. The bipolar transistoris RBJT 73. The parallel diode is distributed diode 74. The RBJT and thedistributed diode of the assembly are parts of the same integratedcircuit 78. The first package terminal is package terminal 70. Thesecond package terminal is package terminal 71. Assembly involvessurface mounting the current transformer and the integrated circuit tothe heat sink portion of the leadframe, and then wire bonding thecomponents together as illustrated in FIG. 27, and then overmolding thecomponents to realize the finished packaged electronic deviceillustrated in FIG. 28.

FIG. 29 is a simplified flowchart of a method 300 in accordance with onenovel aspect. A LFVR is fabricated (step 301) by assembling a currenttransformer, a bipolar transistor, and a parallel diode such that thecurrent transformer supplies an adequate base current to the transistor(when a current greater than I_(C-CRIT) flows from the first packageterminal of the LFVR to the second package terminal of the LFVR) thatthe transistor has a collector-to-emitter voltage substantially lessthan 0.7 volts (for example, 0.1 volts).

Although certain specific embodiments are described above forinstructional purposes, the teachings of this patent document havegeneral applicability and are not limited to the specific embodimentsdescribed above. Although the LFVR is described above in an applicationinvolving a specific flyback converter power supply, the LFVR is ofgeneral applicability in other circuits including other power supplycircuit topologies. Although an example is set forth above where thebase current injection circuit is implemented using a currenttransformer, the base current injection circuit can be implemented inother ways. Accordingly, various modifications, adaptations, andcombinations of various features of the described embodiments can bepracticed without departing from the scope of the invention as set forthin the claims.

1-24. (canceled)
 25. A method comprising: assembling a currenttransformer, a bipolar transistor and a diode such that a first end of afirst winding of the current transformer is coupled to a first packageterminal of a package, such that a first end of a second winding of thecurrent transformer is coupled to the first package terminal of thepackage, such that a second end of the first winding of the currenttransformer is coupled to a base of the bipolar transistor, such that asecond end of the second winding of the current transformer is coupledto a collector of the bipolar transistor, such that an emitter of thebipolar transistor is coupled to a second package terminal of thepackage, such that an anode of the diode is coupled to the collector ofthe bipolar transistor, such that a cathode of the diode is coupled tothe emitter of the bipolar transistor, and such that the packageincludes an amount of molded plastic encapsulant that encapsulates thecurrent transformer, the bipolar transistor and the diode.
 26. Themethod of claim 25, wherein the bipolar transistor and the diode areparts of an integrated circuit.
 27. The method of claim 25, wherein thepackage has no package terminal other than the first package terminaland the second package terminal.
 28. The method of claim 25, wherein theanode of the diode is coupled to the collector of the bipolar transistorvia a collector metal electrode.
 29. The method of claim 25, wherein thediode is a distributed diode that comprises a plurality of PN junctions.30. The method of claim 25, wherein the bipolar transistor comprises abase metal electrode that adjoins the base, wherein the bipolartransistor comprises a collector metal electrode that adjoins thecollector, and wherein the collector metal electrode bridges over thebase metal electrode.
 31. The method of claim 25, wherein there is nodiode whose anode is directly connected to the emitter of the bipolartransistor and whose cathode is directly connected to the base of thebipolar transistor.
 32. The method of claim 25, wherein the firstwinding has a first number of turns, wherein the second winding has asecond number of turns, and wherein the first number is at least twiceas large as the second number.
 33. The method of claim 25, wherein thebipolar transistor has a reverse breakdown voltage between the emitterof the bipolar transistor to the base of the bipolar transistor, andwherein the reverse breakdown voltage is at least twenty volts.
 34. Themethod of claim 25, wherein the current transformer has a ferrite core.35. A method comprising: coupling an anode of a diode to a collector ofa bipolar transistor; coupling a cathode of the diode to an emitter ofthe bipolar transistor; and packaging the diode, a current transformerand a bipolar transistor in a two-terminal package, wherein a first endof a first winding of the current transformer is coupled to a firstpackage terminal of the package, wherein a first end of a second windingof the current transformer is coupled to the first package terminal ofthe package, wherein a second end of the first winding of the currenttransformer is coupled to a base of the bipolar transistor, wherein asecond end of the second winding of the current transformer is coupledto a collector of the bipolar transistor, wherein an emitter of thebipolar transistor is coupled to a second package terminal of thepackage, and wherein the package includes an amount of molded plasticencapsulant that encapsulates the diode, the current transformer and thebipolar transistor.
 36. The method of claim 35, wherein the bipolartransistor and the diode are parts of an integrated circuit.
 37. Themethod of claim 35, wherein the anode of the diode is coupled to thecollector of the bipolar transistor via a collector metal electrode. 38.The method of claim 35, wherein the diode is a distributed diode thatcomprises a plurality of PN junctions.
 39. The method of claim 35,wherein the bipolar transistor comprises a base metal electrode thatadjoins the base, wherein the bipolar transistor comprises a collectormetal electrode that adjoins the collector, and wherein the collectormetal electrode bridges over the base metal electrode.
 40. The method ofclaim 35, wherein there is no diode whose anode is directly connected tothe emitter of the bipolar transistor and whose cathode is directlyconnected to the base of the bipolar transistor.
 41. The method of claim35, further comprising: wrapping the first winding and the secondwinding around a ring-shaped core.
 42. The method of claim 35, whereinthe current transformer has a ferrite core.
 43. The method of claim 35,wherein there is no resistive current sense element disposed in acollector current path between the first package terminal and thecollector of the bipolar transistor.